SMALL PYRAMIDAL NEURON OF RAT CEREBRAL CORTEX - SYNAPSES UPON DENDRITIC SPINES

The pyramidal neurons in layers II and III of the rat parietal cortex have dendritic spines which form synapses with axon terminals. These synapses have synaptic clefts containing granular material that is concentrated towards the middle of the cleft to form a plaque. Only a small amount of dense material occurs on the cytoplasmic face of the presynaptic membrane, while there is a prominent dense layer, some 300 Å deep, in the dendritic spine. When the synapses formed by the smallest dendritic spines are examined in a frontal or en face plane of section this postsynaptic density has the form of a disc. In the synapses on larger spines, the disc is perforated to form a ring, and in the largest spines a number of perforations may occur. Because of these perforations, in larger synapses sections passing at right angles to the plane of the synaptic junction may show two or more separate postsynaptic densities. The possible significance of these findings is discussed. © 1969 Springer-Verlag.